1. Field of the Invention
The present invention relates to a belt type continuously variable transmission for a straddle type vehicle.
2. Description of Related Art
Straddle type vehicles provided with a belt type continuously variable transmission are known. The belt type continuously variable transmission comprises a primary sheave, to which driving force from an engine is transmitted, and a secondary sheave, to which driving force is transmitted through a V-belt from the primary sheave. A speed reduction ration varies in accordance with a ratio of a diameter, at which a belt is wound round the primary sheave, and a diameter, at which the belt is wound round the secondary sheave.
Belt type continuously variable transmissions are usually used in a “dry environment” in which oil, moisture content, etc. do not enter. In order to favorably transmit driving force, a frictional force of a certain measure is needed between a sheave and a V-belt. However, states of contact/non-contact are switched over between the V-belt and the sheave as the V-belt runs, so that some lubricating ability is needed between the sheave and the V-belt. With no lubricating ability, heat is generated due to friction between the V-belt and the sheave.
It is thought that lubricating ability between the sheave and the V-belt is provided mainly by abrasion powder generated from the V-belt. Abrasion powder generated by contact with a sheave is held on a portion of the sheave in contact with the V-belt whereby the sheave and the V-belt slide relative to each other while maintaining a frictional force of a certain measure. When the sheave surface is smooth, however, the abrasion powder cannot be suitably held on the sheave surface and it becomes difficult to preserve a lubricating ability between the sheave and the V-belt. Accordingly, irregularities are needed on the sheave surface to hold the abrasion powder.
However, irregularities on the sheave surface make the V-belt liable to abrade. Therefore, when the irregularities are excessively large in magnitude, the service life of the V-belt is decreased. Thus, the irregularities are preferably limited in magnitude to a predetermined range.
Patent Gazette No. 3609754 describes that if the portion of the V-belt that is in contact with the sheave surface is formed from a resin, abrasion loss of the V-belt is reduced by making surface roughness of the sheave surface Ra 0.5 to 3.0 μm.
However, the present inventors have determined that even when surface roughness of the sheave surface is in this range (Ra 0.5 to 3.0 μm), abrasion loss of the V-belt is not necessarily reduced. That is, even when the surface roughness of the sheave surface is in this range, some V-belts have a small amount of abrasion loss while other V-belts have a large amount of abrasion loss.